REGULATION OF BP 2/24/2019 Regulation of BP

Systemic Arterial Pressure Normal range is about 120/80 mmHg Varies with age: 17 – 40  140/90mmHg 41 – 60  150/90 > 60  160/90 Higher systolic pressure with age reflects in part diminishing arterial compliance Pulse pressure = SBP –DBP 2/24/2019 Regulation of BP

Physiological changes in BP Mean Arterial pressure (MAP) Average of all the pressures measured mSec. By mSec. Over a period of time. MAP = DBP + 1/3 pulse pressure 2/24/2019 Regulation of BP

Physiological changes in BP MAP is determined by CO and systemic vascular resistance MAP =CO x TPR CO and TPR determine average volume of blood in systemic arteries over time. It is this blood that causes pressure. 2/24/2019 Regulation of BP

Physiological changes in BP It is total arteriolar resistance that influence systemic arterial pressure Clinical methods for measuring BP Korotkoff sounds Arterial pressure determine Blood flow Decreased BP → Hypoperfusion Increased BP → i.e need for control 2/24/2019 Regulation of BP

Regulation of BP Autoregulation Homornal regulation Nervous system Renal Note Short term vs long term regulation 2/24/2019 Regulation of BP

Regulation of local blood flow 2/24/2019 Regulation of BP

Blood flow to different organs and tissues under basal conditions. % ml/min ml/min/100gm Brain 14 700 50 heart 4 200 70 Brounchi 2 100 25 Kidney 22 1100 360 Liver 27 1350 95 Muscle 15 750 4 (inactive state) bones 5 250 3 Myroid gland 1 50 160 Adrenal gland 0.5 25 300 Skin (cool) 6 300 3 Other tissues 3.5 175 1.3 2/24/2019 Regulation of BP

Regulation of local blood flow Autoregulation ability of tissue to regulate its own blood flow ↑ tissue metabolism Or ↓ O2 availability →↑local blood flow How Vasodilator/metabolic theory O2 demand/myogenic theory 2/24/2019 Regulation of BP

Regulation of local blood flow In any tissue of the body acute increase in arterial pressure will cause an immediate rise in blood flow, the blood flow in most tissues return most of the way back towards the normal level How Metabolic theory Myogenic theory 2/24/2019 Regulation of BP

Regulation of local blood flow dilatation of large arteries when microvascular blood flow is increased. EDRF 2/24/2019 Regulation of BP

Regulation of local blood flow Chronic changes in local blood flow leads to Change in tissue vascularity Neovascularization/Angiogenesis Angiogenic factor released by Ischaemic tissues Rapidly growing tissues Tissues with excessively high met 2/24/2019 Regulation of BP

Systemic regulation of BP by hormones vasoconstrictors Vasopressin Norepinephrine & Epinephrine Angiotensin II vasodilators Bradykinins formed during active secretions in sweat glands salivary glands & pancrease. ANP Secreted by the heart antagonizes the action of various vasoconstrictors and lowers BP. 2/24/2019 Regulation of BP

Regulation of BP by the NS 2/24/2019 Regulation of BP

Regulation of BP by the NS Nervous control of BP is by far the most rapid of all mechanisms for pressure control. The means by which the NS controls BP is almost entirely through the Autonomic NS. The most important part of ANS for regulation of BP is SNS. 2/24/2019 Regulation of BP

Regulation of BP by the NS All vessels except capillaries, precapillary sphincters and most of metarterioles are innervated by SNS . 2/24/2019 Regulation of BP

In the medulla and lower third of pons. Vasomotor Center: In the medulla and lower third of pons. The center transmits parasympathetic impulses through the vagus nerves to the heart, and sympathetic impulses through the cord and peripheral nerves to blood vessels. 2/24/2019 Regulation of BP

Vasomotor center Sympathetic chain heart 2/24/2019 Regulation of BP

Vasoconstrictor area (C1 Vasodilator area (A1) Vasomotor Center Vasoconstrictor area (C1 Nerves secrete Noradrenaline. Vasodilator area (A1) whose fibers project to the C1 and inhibit vasoconstrictor activity of C1   2/24/2019 Regulation of BP

Vasomotor Center Sensory area (A2) Located in the NTS Neurons of this area receive sensory nerve signals mainly from X and 1X the output signals from this area control the activities of both C1 and A1 2/24/2019 Regulation of BP

IX NTS C1 Carotid sinus X Aortic arch Adrenal medulla heart Vessel 2/24/2019 Regulation of BP

Vasomotor Center Under normal conditions the C1 of the vasomotor center transmits signals continuously to the sympathetic vasoconstrictor nerve fibers over the entire body causing slow firing of these fibers. “Sympathetic vasoconstrictor tone”. These impulses maintain a partial state of contraction in the blood vessels called vasomotor tone. 2/24/2019 Regulation of BP

BP is adjusted by variation in the rate of this tonic discharge. Vasomotor Center BP is adjusted by variation in the rate of this tonic discharge. 2/24/2019 Regulation of BP

Vasomotor Center     Lateral portion of vasomotor center transmit excitatory impulses through the sympathetic nerve fibers to the heart. Medial portions transmit impulses through the vagus nerves to the heart. Sympathetic impulses are transmitted to the adrenal medullae at the same time they are transmitted to the blood vessels. Nerve endings of vasoconstrictor nerve secrete noradrenaline 2/24/2019 Regulation of BP

Vasomotor Center Vasomotor center is controlled by higher nervous centers. Reticular substances of the pons, mesencephalon, diencephalons can excite or inhibit the vasomotor center Cerebral cortex, hypothalamus When vasoconstrictor tone is increased, there is arteriolar constriction and increase in BP. 2/24/2019 Regulation of BP

Vasomotor Center Venoconstriction leads to decrease stores of blood in venous reservoirs usually accompany these changes HR and stoke volume increase because of sympathetic activity, and CO is increased. 2/24/2019 Regulation of BP

Afferents to the vasomotor center. Fibers from baroreceptor Other parts of the NS Carotid and aortic chemoreceptors. Some stimuli act directly on the center, eg CO2, hypoxia.   2/24/2019 Regulation of BP

Baroreceptors: Abundant in the carotid Sirius and arch of the aorta. Signals from the carotid Sinus are transmitted though Hering’s nerve To 1X and then to NTS in the medullary area. 2/24/2019 Regulation of BP

 Baroreceptor  From arch of the aorta to X also into the same area of the medulla. Carotid Sinus baroreceptors are stimulated by pressure more than 60mmHg, reach a maximum stimulation at 180mmHg Baroreceptors respond extremely rapidly to changes in arterial pressure.   2/24/2019 Regulation of BP

Baroreceptor Signals from baroreceptors are carried by 1X/X to NTS Then to Sensory area (A2) And eventually inhibit vasoconstrictor center of the medulla and excite vagal center 2/24/2019 Regulation of BP

Baroreceptor Excitation of baroreceptors reflexly lowers BP Low pressure has opposite effect Baroreceptors opposes either rise or fall n BP, thus called Pressure buffer. Baroreceptors rest in 1 – 2 days to whatever pressure level they are exposed. 2/24/2019 Regulation of BP

Baroreceptor Atrial and Pulmonary Arteries stretch receptors(Low pressure receptors) Detect BP caused by volume. Atrial reflexes to the kidney: Stretch of atrial causes reflex dilatation of afferent arteries to the kidney Leading to decreased secretion of vasopressin 2/24/2019 Regulation of BP

Chemoreceptors Carotid bodies in the bifurcation of each common carotid artery And Aortic bodies adjusent to the aorta. Stimulated when arterial pressure falls below 80mmHg. 2/24/2019 Regulation of BP

CNS ischaemic response. Stimulated when arterial pressure falls below 60mmHg Greatest degree of stimulation at 15 – 20 mmHg. 2/24/2019 Regulation of BP

Long term regulation of BP 2/24/2019 Regulation of BP

Long term regulation of BP When the arterial pressure changes slowly over hrs Or days, the nervous mechanisms gradually lose all or almost all of their ability to oppose the changes. 2/24/2019 Regulation of BP

Long term regulation of BP Renal-body fluid system Renin Angiotensin system 2/24/2019 Regulation of BP

Renal – body fluid system ↑ ECF →↑arterial pressure Rising pressure in turn has a direct effect to cause the kidneys to excrete the excess ECF, i.e. returning pressure to normal This is the basis for long term arterial pressure control. 2/24/2019 Regulation of BP

9 -- 8 -- 7 -- 6 -- 5 -- Urinary volume output 4 -- 3 -- 2 -- 1 -- ( X normal) 50 100 150 200 Arterial pressure (mmHg) 2/24/2019 Regulation of BP

Renal – body fluid system Determinants of long term Arteriolar pressure are: Renal output of salt and water Salt and water intake 2/24/2019 Regulation of BP

How increase in ECF vol. raise BP Increase in ECF volume increases blood volume Increasing Mean Systemic Filling pressure Increase venous return Increase CO Autoregulation causes increase in TPR Raising arterial pressure 2/24/2019 Regulation of BP

Note that: Hypertension is often caused by excessive ECF volume ECF volume  CO  Autoregulation Autoregulation (TPR)  BP 2/24/2019 Regulation of BP

Renin-Angiotensin System Apart from capability of control of arterial pressure through changes in ECF Vol. The kidneys have another powerful mech. Renin is small protein enzyme released by the kidney When arterial pressure falls too low. Or reduced ECF Vol. Synthesized and stored in inactive form in JG cell which are modified smooth muscle cells in the afferent arterioles 2/24/2019 Regulation of BP

Angiotensinogen  Angiotensin I (plasma protein) Arterial pressure  Renin Angiotensinogen  Angiotensin I (plasma protein) ACE (lungs) Angiotensin II Angiotensinase Inactivated Renal retention of salt&water vasoconstriction BP  2/24/2019 Regulation of BP

Effects of Angiostensin II Vasoconstriction occurs rapidly Intensely in arterioles Less extent in veins Max. effect after 20 – 30 min Act on the kidney to reduce salt and water excretion. This slowly increases ECF Volume which then raises arterial pressure over hrs – days. 2/24/2019 Regulation of BP

Effects of Angiostensin II Acts directly on kidneys Or to the Adrenal cortex to secrete Aldosterone 2/24/2019 Regulation of BP

In the kidneys Constrict renal blood vessels, diminishing blood flow i.e. less fluid filters through the glomeruli into the tubules Slows blood flow in peritubular capillaries Reducing their pressure which allows rapidly osmotic reabsoption of fluid from the tubules. Effect on tubular cells themselves to increase tubular rabsorption of salt and water. Aldosterone causes marked increase in Na+ and water reabsoption by renal tubules. 2/24/2019 Regulation of BP

Effects of Angiostensin II Acts on brain to increase water intake Secretion of ADH Facilitate release of Naradrenaline by direct action on postanganglionic sympathetic nerves 2/24/2019 Regulation of BP

Renin Angiotensin System Increase in ECF Vol. Raises BP Resulting in decreased Renin release( i.e decreased Angiotensin II) Therefore less renal reabsorption of salt and water 2/24/2019 Regulation of BP